For many purposes in agriculture and related endeavors it is desired to treat plants with exogenous chemical substances of various kinds. Many exogenous chemical substances are applied to foliage (i.e., leaves and other non-woody above-ground parts) of a plant, and have a site of action in the plant either close to or remote from the locus of application. Such substances are referred to herein as foliar-applied exogenous chemical substances.
Typically, when an exogenous chemical substance is applied to foliage by plant treatment processes known in the art, only a small portion of the amount applied reaches sites of action in the plant where a desired biological activity of the exogenous chemical substance can be usefully expressed. It is therefore a major desideratum in agriculture and related endeavors to enhance the efficiency of delivery of foliar-applied exogenous chemical substances to their sites of action in plants, and thereby to enhance the biological effectiveness of the exogenous chemical substance for the purpose for which the exogenous chemical substance is used.
Application to foliage of an exogenous chemical substance by processes known in the art does not universally result in inefficient delivery to sites of action. In some situations such processes provide excellent biological effectiveness, even at a low use rate of the exogenous chemical substance. In other situations the same processes, using the same rate of the exogenous chemical substance, provide inadequate biological effectiveness. Thus, these processes are inconsistent in the result they provide, or they cannot be relied upon to provide the desired result.
A problem is that it is seldom possible to identify in advance those situations where good biological effectiveness will be obtained, partly because so many factors influence efficiency of delivery. These factors include weather (temperature, relative humidity, daylength, cloudiness, precipitation, wind, etc.) preceding, during and following application, soil conditions (fertility, aeration, etc.), plant growth stage, health and physiological status, equipment-related inaccuracies in application, and other factors. Therefore, to help ensure reliable or consistent biological effectiveness of a foliar-applied exogenous chemical substance, the user typically applies the substance at a higher rate than truly necessary in the majority of situations.
Variability in biological effectiveness in field conditions is an especially troublesome problem in the case of exogenous chemical substances that are acids, and are typically formulated as water-soluble salts in which the exogenous chemical substance is present in an anionic form. Sometimes by converting such acid substances to esters, this variability can be moderated; however, in many cases esters show reduced biological effectiveness, for example due to inadequate conversion back to the parent acid once inside the treated plant. There remains a strong need for enhanced biological effectiveness, and enhanced reliability of biological effectiveness, of foliar-applied exogenous chemical substances, particularly anionic exogenous chemical substances.
The term "anionic exogenous chemical substance" as used herein means an exogenous chemical substance whose molecular structure includes one or more acid, or proton-donating, sites, and is therefore capable of forming an anion in the presence of a proton acceptor. The term therefore embraces substances that are zwitterionic. In describing an exogenous chemical substance as "anionic" herein, it is not implied that the exogenous chemical substance is necessarily in anionic form or that it is dissociated.
Benefits of a process providing greater reliability of biological effectiveness include an ability to reduce rates of application of exogenous chemical substances without sacrificing consistency of biological effectiveness. Pressures felt by the agricultural industry to reduce pesticide, particularly herbicide, usage are well evidenced by symposia on the subject, such as that held in 1993 by the Weed Science Society of America and documented in Weed Technology 8, 331-386 (1994). Reduced use rates bring rewards not only environmentally but also economically, as the cost per unit area treated decreases.
Foliar-applied exogenous chemical substances have frequently been applied together with amphiphilic materials, particularly amphiphilic surface-active agents, otherwise known as surfactants. Surfactants can influence biological effectiveness of a foliar-applied exogenous chemical substance in numerous ways.
When a dilute aqueous composition of an exogenous chemical substance is applied to foliage by conventional hydraulic spraying, the presence of surfactant in the dilute aqueous composition can alter the size distribution of the spray droplets, typically increasing the percentage of spray volume in the form of small droplets and reducing the percentage of spray volume in the form of large droplets. As smaller droplets have lower momentum than larger droplets, these smaller droplets are less likely to rebound from a foliar surface and consequently are more likely to be retained on that surface. Spray retention can also be facilitated by adhesion between surfactant molecules in a spray droplet and the foliar surface, which in most plants is waxy and hydrophobic. This adhesion reduces not only rebound but also run-off of spray droplets from the foliar surface. Surfactants also tend to increase the area of contact between a spray droplet and a foliar surface, and in many cases enhance penetration of an exogenous chemical substance from the droplet into and through cuticles of leaves to reach internal leaf tissues.
Through these and perhaps other effects, amphiphilic materials including surfactants have long been known to increase the biological effectiveness of exogenous chemical substances. It is therefore commonplace for one or more surfactants to be included in commercial formulations of foliar-applied exogenous chemical substances, even in formulations that do not require the presence of surfactants for acceptable physical stability or handling properties, for example as emulsifying or suspending agents or dispersants.
One of the most extensively studied of foliar-applied anionic exogenous chemical substances, from the point of view of the role of surfactants in enhancing biological effectiveness, is the herbicide glyphosate. As well as being a phytotoxic agent, glyphosate has been used as a plant growth regulator.
Glyphosate in its strict sense is an acid compound, N-phosphonomethylglycine, but the word "glyphosate" is herein used in a less restrictive sense, except where the context dictates otherwise, to encompass not only glyphosate acid but also salts, adducts and esters thereof, and compounds which are converted to glyphosate in plant tissues or which otherwise provide glyphosate ions. In most commercial formulations of glyphosate, the glyphosate is present as a water-soluble salt. In this respect, glyphosate is typical of most exogenous chemical substances that are acids or that form anions.
Herbicidal salts of glyphosate are disclosed, for example, in U.S. Pat. No. 3,799,758 to Franz, U.S. Pat. No. 3,853,530 to Franz, U.S. Pat. No. 4,140,513 to Prill, U.S. Pat. No. 4,315,765 to Large, U.S. Pat. No. 4,405,531 to Franz, U.S. Pat. No. 4,481,026 to Prisbylla and U.S. Pat. No. 4,507,250 to Bakel. In most of the salts disclosed, the counterion to glyphosate anion is a relatively low molecular weight, non-amphiphilic cation. Typical of such salts are alkali metal, for example sodium and potassium, salts; ammonium salt; and numerous salts having an ammonium, sulfonium or sulfoxonium cation substituted with 1-3 organic groups containing in total 1-6 carbon atoms, for example dimethylammonium, isopropylammonium, ethanolammonium and trimethylsulfonium salts.
Commercial formulations of glyphosate salts include, for example, Roundup.RTM.) brand, Accord.RTM. brand and Roundup.RTM. Ultra brand herbicides of Monsanto Company, which contain the isopropylammonium salt, Roundup.RTM. Dry brand and Rival.RTM. brand herbicides of Monsanto Company, which contain the ammonium salt, Roundup.RTM. Geoforce brand herbicide of Monsanto Company, which contains the sodium salt, and Touchdown.RTM. brand herbicide of Zeneca, which contains the trimethylsulfonium salt.
Salts of glyphosate with higher molecular weight, amphiphilic cations have also been disclosed. Such amphiphilic cations include those having a hydrophilic moiety such as an ammonium, ethanolammonium, polyoxyethylene ammonium, or sulfonium group, and a hydrophobic moiety comprising 1 to 4 hydrocarbyl groups having in total more than 6 carbon atoms. For example, above-cited U.S. Pat. No. 4,405,531, the disclosure of which is incorporated herein by reference, discloses a wide range of primary, secondary and tertiary organic ammonium salts of glyphosate wherein the cation is amphiphilic as defined immediately above and has a molecular weight of less than about 300. International Publication No. WO 83/03608, European Patent Application No. 0 124 351 and U.S. Pat. No. 4,431,594 disclose various quaternary ammonium salts of glyphosate wherein the cation is amphiphilic. U.S. Pat. No. 5,668,085 discloses salts of glyphosate with amphiphilic cations derived from polyoxyethylene tertiary C.sub.8-22 alkylamine surfactants, a specifically disclosed example being the N-cocoalkyl-N,N-diethanolammonium salt of glyphosate where "cocoalkyl" refers to a mixture of predominantly C.sub.12 and C.sub.14 alkyl chains, derived from coconut oil. In all organic ammonium salts of glyphosate specifically identified in the above disclosures, the cation has only one nitrogen-containing group, namely an amino or ammonium group, although above-cited U.S. Pat. No. 4,405,531 generically discloses salts prepared from low molecular weight organic amines "containing not more than 2 amine groups".
Glyphosate as a herbicide has many advantages, particularly environmental advantages including biodegradability and low ecotoxicity. However, studies have shown that even the most biologically effective formulations of glyphosate presently in use do not deliver glyphosate efficiently to sites in the plant where the glyphosate exerts its phytotoxic effect. Typically, only a small fraction of the applied herbicide arrives at such sites. The term "biodisponibilite" (approximately equivalent in meaning to the English word "bioavailability") is used in French patent application no. 97-08371 from which the present application claims priority, to refer to this fraction.
The small fraction of applied glyphosate which reaches sites of phytotoxic action is related to the fact that the glyphosate must go through several barriers. Among these, one of the most important is believed to be the lipophilic cuticle on the foliar surface to which the glyphosate is applied. It has therefore been theorized that it would be desirable to place the glyphosate into an amphiphilic medium which would provide greater compatibility between the lipophilic cuticle and the hydrophilic glyphosate, and thereby facilitate penetration of glyphosate into and through the cuticle. Similar thinking has been applied to other exogenous chemical substances, particularly those typically formulated as water-soluble salts.
That the concept of an amphiphilic medium as an aid to cuticular penetration and thereby enhanced biological effectiveness, for example of glyphosate, has validity is demonstrated by many studies in which foliar uptake or effectiveness has been enhanced by surfactants. An extensive study by Wyrill & Burnside, Weed Science 25, 275-287, 1977 led to a conclusion that "an effective surfactant is a critical component of any glyphosate spray mixture", but noted great variation among surfactant types in the degree of enhancement of herbicidal effectiveness afforded. In general, cationic surfactants gave greater enhancement than nonionic surfactants. Data are reported in International Publication No. WO 98/06259 for a wide range of cationic, nonionic, anionic and amphoteric surfactants applied either in mixture with, or in sequence following, a glyphosate composition.
Another approach to providing an amphiphilic medium has been to apply glyphosate together with a lipophilic agent, such as an oil, in the form of a water-in-oil emulsion or microemulsion. Such emulsions or microemulsions are disclosed in European Patent Application No. 0 379 852, U.S. Pat. No. 4,853,026 and U.S. Pat. No. 5,248,086. A disadvantage of such microemulsions is that, when provided as concentrate compositions, they are subject to the phenomenon of breaking of the emulsion upon dilution with water to concentrations suitable for application, for example, 5 grams of glyphosate, expressed as acid equivalent, per liter (g a.e./l). In other words, water-in-oil microemulsions tend not to withstand dilution in water. The failure of such microemulsions to provide improved cuticular penetration is perhaps related to this inability to withstand dilution.
Oil-in-water macroemulsion formulations of glyphosate have also been investigated. In these macroemulsions, the majority of the glyphosate is present in the continuous aqueous phase, as shown, for example, in European Patent Application No. 0 485 207. Such macroemulsions, in which the glyphosate and the lipophilic component are segregated, do not therefore provide glyphosate in an amphiphilic form, and have generally not enhanced delivery of glyphosate to its sites of phytotoxic action in the plant.
A different approach, illustrated in European Patent Specification No. 0 148 169, is to encapsulate a water-soluble herbicide such as glyphosate in a polymeric shell by interfacial polycondensation. In this technique, a water-in-oil emulsion having a lipophilic emulsifier based on alkylated polyvinylpyrrolidone is used. Polymerization to form the shell, by reaction of comonomers, occurs at the oil-water interface of the emulsion containing the herbicide, resulting in formation of a shell that encapsulates the herbicide.
All of the approaches summarized above, including formulating an anionic exogenous chemical substance as an amphiphilic salt, have met with limited success in overcoming the barriers to delivery of the exogenous chemical substance to its sites of biological action in the plant. It is an objective, therefore, of the present invention to provide a new composition or formulation of an exogenous chemical substance, in particular an anionic exogenous chemical substance, that can provide superior biological effectiveness when applied to foliage of a plant.
Another objective of the invention is to provide a composition or formulation of an exogenous chemical substance, in particular an anionic exogenous chemical substance, that is economical and simple to make.
Another objective of the invention, particularly as it applies to the herbicide glyphosate, is to provide a composition or formulation that meets the previously stated objectives while permitting maintenance of the non-ecotoxic and biodegradable character of glyphosate.
Another objective of the invention is to provide a composition or formulation of an exogenous chemical substance, particularly an anionic exogenous chemical substance, that can be applied in a dilute aqueous medium and does not lose its beneficial properties at high rates of dilution.
Another objective of the invention is to provide an aqueous composition or formulation of an anionic exogenous chemical substance in the form of an amphiphilic salt that is physically stable, even at high concentration, without the need for additional stabilizing agents such as dispersants or emulsifying agents.
Another objective of the invention is to provide a convenient and economical method for the preparation of a composition or formulation that meets the objectives stated above.
These and other objectives have been satisfied through design of a new approach for promoting transport of an anionic exogenous chemical substance into plants via foliage, and thereby promoting biological effectiveness of the exogenous chemical substance. This approach, as set out more fully below, involves the production of a colloidal dispersion of supramolecular aggregates, or nanoparticles, containing the exogenous chemical substance wholly or partly in the form of an amphiphilic salt thereof.